1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory and a method of manufacturing the same, and particularly relates to a nonvolatile semiconductor memory having a NAND-type memory cell unit structured by connecting a plurality of memory cell transistors in series and a method of manufacturing the same.
2. Related Background Art
An EEPROM which enables electrical rewrite has been hitherto known as one of semiconductor memories. Especially, a NAND-type EEPROM in which a NAND-type memory cell unit is structured by connecting a plurality of memory cell transistors in series has attracted considerable attention as one capable of high integration.
FIG. 41 is a diagram showing an equivalent circuit of the NAND-type memory cell unit, and FIG. 42 is a diagram showing the structure of a memory cell portion of the NAND-type memory cell unit in plan view. This example in FIG. 42 shows the NAND-type memory cell unit in the case where STI (Shallow Trench Isolation) is used for isolating elements.
One memory cell transistor MT of the NAND-type EEPROM has an FETMOS structure in which a floating gate FG (a charge storage layer) and a control gate CG are stacked with an insulating film therebetween above a semiconductor substrate, a plurality of memory cell transistors are connected in series with their respective adjoining ones sharing a source/drain to constitute the NAND-type memory cell unit. The NAND-type memory cell units like this are arranged in a matrix form to constitute a memory cell array.
A drain D at one end side of the NAND-type memory cell unit is connected to a bit line BL via a select gate transistor ST31, while a source at the other end side of the NAND-type memory cell unit is connected to a common source line SL via a select gate transistor ST32. The control gate CG of the memory cell transistor MT and gate electrodes of the select gate transistors ST31 and ST32 are respectively connected to compose a word line WL and a select gate line in a direction perpendicular to the direction of the bit line BL.
As shown in FIG. 42, in the NAND-type EEPROM, one source/drain line formed out of a diffusion layer in a silicon active region is formed for one bit line BL. Namely, one NAND-type memory cell unit is formed for one bit line BL. Assuming that a design rule is F (Feature size), the line/space of the bit line BL is 1 F/1 F, and the line/space of the word line WL is also 1 F/1 F. Therefore, the cell size of one memory cell transistor MT is 2 Fxc3x972 F=4 F2. Since the select gate transistors ST31 and ST32 are provided in one NAND-type memory cell unit, the substantial one cell size is 4 F2+xcex1 if the sizes of these select gate transistors ST31 and ST32 are taken into account as an overhead xcex1.
As a widely known example of such a NAND-type EEPROM, there are reports such as xe2x80x9cA 3.3 V 32 MB NAND Flash Memory with Incremental Step Pulse Programming Schemexe2x80x9d by K.-D. Suh et al. in IEEE J. Solid-State Circuits, vol. 30, pp. 1149-1156, November 1995 and xe2x80x9cA 35 ns Cycle Time 3.3 V Only 32 MB NAND Flash EEPROMxe2x80x9d by Y. Iwata et. al. in IEEE J. Solid-State Circuits, vol. 30, pp. 1157-1164, 1995. In these references, the operation of a related NAND-type EEPROM is explained.
FIG. 43 is a diagram showing an equivalent circuit of a nonvolatile semiconductor memory having AND-type memory cell units, and FIG. 44 is a diagram showing the structure of a memory cell portion of the AND memory cell unit in plan view.
The name of an AND type originates in that its connection mode is the same parallel connection as an NOR type and that its logic mode is inverse to the NOR type. Namely, as shown in FIG. 43, the AND-type memory cell unit has a sub-bit line SBBL and a sub-source line SBSL, and a plurality of memory cell transistors MT are connected in parallel between the sub-bit line SBBL and the sub-source line SBSL. For example, in the case of a 64 Mbits AND-type nonvolatile semiconductor memory, 128 memory cell transistors are connected in parallel in one AND-type memory cell unit.
The sub-bit line SBBL is connected to a main bit line MBL via a select gate transistor ST41. The sub-source line SBSL is connected to a main source line MSL via a select gate transistor ST42.
A memory cell array composed of these AND-type memory cell units is characterized by its pseudo contactless structure, in which the main bit line MBL and word lines WL are made hierarchical and the sub-bit line SBBL and the sub-source line SBSL are formed out of a diffusion layer writing/erase into/from the memory cell transistor MT is performed by an FN (Fowler-Nordheim) tunnel current. Specifically, the writing to the memory cell transistor MT is performed by extracting electrons in the floating gate FG to the drain side by the use of the FN tunnel current. The erase from the memory cell transistor MT is performed by injecting electrons from the semiconductor substrate to the floating gate FG by the FN tunnel current on the entire surface of a channel.
As shown in FIG. 44, in the AND-type memory cell unit, two lines in total, i.e. the sub-source line SBSL and the sub-bit line SBBL formed of the diffusion layer in the silicon active region, are formed for one main bit line MBL. Hence, the line/space of each of the sub-source line SBSL and the sub-bit line SBBL is 1 F/1 F, and the line/space of the word line WL is also 1 F/1 F. Consequently, the cell size of one memory cell transistor MT is 2 Fxc3x974 F=8 F2. Moreover, since the select gate transistors ST41 and ST42 are provided in one AND memory cell unit, the substantial one cell size is 8 F2+xcex1 if the sizes of these select gate transistors ST41 and ST42 are taken into account as an overhead xcex1.
Meanwhile, Japanese Patent Laid-open No. Hei 7-45797 discloses a nonvolatile semiconductor memory in which a vertical NAND memory cell unit is formed in a side wall portion of a trench in order to reduce one cell size. FIG. 45 is a diagram showing a cross section of a memory cell transistor MT portion of the nonvolatile semiconductor memory disclosed in this Japanese Patent Laid-open No. Hei 7-45797.
As shown in FIG. 45, in this nonvolatile semiconductor memory, a trench region TC is formed on a semiconductor substrate, and memory cell transistors MT are formed respectively on both side wall portions of this trench region TC. In this case, the floating gates FG are formed along a side wall on the inside of the trench region TC, and source/drains SD are formed as a diffusion layer along a side wall of the trench region TC of the semiconductor substrate. Namely, in this NAND-type memory cell unit, a plurality of memory cell transistors MT are each formed along the side wall of the trench region TC, and thus a source/drain current flows along the side wall of the trench region TC. The bit line BL is formed via an interlayer dielectric for each NAND-type memory cell unit. The line/space of this bit line BL is 1 F/1 F.
In order to attain higher integration, however, it is necessary to lay two source-drain lines formed in silicon active regions within a 2 F bit line pitch and to effectively reduce the memory cell size by half.
The present invention is made in view of the aforesaid problems. An object of the present invention is to three-dimensionally make an NAND-type EEPROM and lay two source/drain lines in silicon active regions within 2 F bit line pitch, that is, to provide two NAND-type memory cell units for one bit line, and thereby to provide a nonvolatile semiconductor memory capable of reducing the memory cell size by half, resulting in a reduction in bit cost and to provide a method of manufacturing the same.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a nonvolatile semiconductor memory includes a memory cell array having a plurality of memory cell units, one of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, the nonvolatile memory cell transistors having a first insulating film formed on a substrate; a charge storage layer formed on the first insulating film; a control gate shared with two adjacent transistors which are two of the nonvolatile memory cell transistors and which are adjacent to each other; and a second insulating film formed between the charge storage layer and the control gate, both a bottom of the charge storage layer and a bottom of the second insulating film being on a face which is substantially parallel with a surface of the substrate.
wherein the NAND-type memory cell units are formed in pairs along both side wall portions of a trench formed in a substrate,
two of the nonvolatile memory cell transistors which face each other on the side wall portions of the trench share the one control gate which is formed to extend in a depth direction of the trench, and
the control gate is formed to fill a space formed by an insulating film which covers the two charge storage layers facing each other on the side wall portions of the trench, and electrically connected to a word line which extends continuously.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a nonvolatile semiconductor memory comprising a plurality of NAND-type memory cell units, each of which includes:
a NAND-type memory cell column having a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate;
a bit line side switching portion connected between the memory cell column and a bit line; and
a source line side switching portion connected between the memory cell column and a source line,
wherein the NAND-type memory cell units are formed in pairs along both side wall portions of a trench formed in a substrate, and
wherein two of NAND-type memory cell units of the pair are respectively connected to the identical bit line via the bit line switch.
According to a further aspect of the present invention, a method of manufacturing a nonvolatile semiconductor memory including a memory cell array having a plurality of NAND-type memory cell units, each of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate, the method comprising the steps of:
forming a trench in a substrate;
forming first insulating films on both side wall portions of the trench;
forming a pair of the charge storage layers on surface sides of the first insulating films formed on both the side wall portions of the trench;
forming a second insulating film so as to cover the pair of charge storage layers on surface sides of the pair of charge storage layers formed on both the side wall portions of the trench;
forming the control gate shared by the pair of charge storage layers so as to fill a space formed by the second insulating film; and
forming a word line which is electrically connected to the control gate and extends continuously.
According to a still further aspect of the present invention, a method of manufacturing a nonvolatile semiconductor memory including a memory cell array having a plurality of NAND-type memory cell units, each of which has a plurality of rewritable nonvolatile memory cell transistors connected in series, each of which has a charge storage layer and a control gate, the method comprising the steps of:
forming a trench in a substrate;
forming first insulating films for memory cell transistors on both side wall portions of the trench;
forming first insulating films for select gate transistors on both side wall portions of the trench;
forming a pair of the charge storage layers on surface sides of the first insulating films for the memory cell transistors formed on both the side wall portions of the trench;
forming a pair of first gate electrodes on surface sides of the first insulating films for the select gate transistors formed on both the side wall portions of the trench;
forming a second insulating film so as to cover the pair of charge storage layers on surface sides of the pair of charge storage layers formed on both the side wall portions of the trench;
forming the control gate shared by the pair of charge storage layers so as to fill a space formed by the second insulating film;
forming a second gate electrode shared by the pair of first gate electrodes so as to fill a space between the pair of first gate electrodes;
forming a word line which is electrically connected to the control gate and extends continuously; and
forming a select gate line which is electrically connected to the second gate electrode and extends continuously.